Miners in the main shard can grief validators in a child shard by censoring collation headers, i.e. not allowing them to be added to the VMC. (The period is only 5 blocks. The loose periods proposal only slightly mitigates the attack.)
Missing the period is costly for individual validators, and this can be used as leverage by miners to extort validators for profit (blackmail) or discourage them from participation. My proposal is to reward validators collation subsidies (not transaction fees) even if they miss adding a collation header in their period.
Site note: I got this idea to address the collation body withdrawal attack in this scheme to separate proposers and validators. Effectively a backup empty collation is pushed at every period by default, and there is no data availability problem for this backup collation.)